Manufacturers of products that are produced in high volume as part of a process using, for example, a process line, employ quality assurance methods to ensure that certain features of the product (e.g., color, pattern, alignment, texture) are consistent and match a production reference standard. For example, in the soda can industry, the patterns and colors on the outer surface of the cans should be monitored somehow as the cans proceed down a process line to ensure that the process of printing the outer surface of the cans does not result in out of tolerance conditions (e.g., color drift, pattern alignment drift, etc.). The product moving down a process line is often spatially oriented in a random manner along the process line. For example, soda cans having a specific pattern printed on the cylindrical outer surface are typically oriented randomly about the vertical axis of rotation of the predominantly cylindrical can.
These methods can be as simple as a production floor operator performing a set-up of a product run by making visual comparison of a finished set-up part to a standard reference chart or reference part. Based on this comparison the operator makes adjustments to the process. Then another set-up part is created and compared, more adjustments made until acceptable results are achieved, and the product run is initiated. This subjective method may lead to errors because of differences in the ambient light conditions, positions of the inspection light source, and differences in surface textures between the reference part and the finished part, different people conducting the comparisons, and other factors. While such a subjective comparison may be appropriate for some manufacturing processes, other more sophisticated processes (e.g., multi-color processes) may require more objective techniques.
Examples of such processes include package printing processes, soda can printing processes, and other processes which may employ more complex color schemes that are repeated or are placed next to each other in use. Besides merely color concerns, these complex color schemes may have spatial or pattern defects. A trained quality assurance color inspector using a standard illuminant may be able to catch many of these defects by using a subjective comparison with a standard reference part, however, many of such defects may not be discernible to the naked eye. In such applications, manufacturers have typically used a color densitometer, a tristimulus colorimeter, or a reflectance spectrophotometer to provide more precise color matching by utilizing colorimetry, discussed in more detail below.
The process of quantitative color analysis is generally referred to as colorimetry. Since the introduction of the CIE (Commission International de l'Eclairage) color measurement system in the early 1930's, many different measurement systems have been proposed for different applications. One such measurement system is the CIE XYZ color space. The CIE XYZ color space characterizes colors by a luminance parameter Y and two color coordinates X and Z which specify the point on the chromaticity diagram. The XYZ parameters are based on the spectral power distribution of the light emitted from a colored object and are factored by sensitivity curves which have been measured for the human eye. The human eye has three different types of color-sensitive cones. Accordingly, the XYZ functions were intended to correspond to the average sensitivity of the human eye and provide a device-independent representation of color. Therefore, the spectral responses of the XYZ functions are known as “tristimulus” functions and make up the coordinate system to quantify a color image or color space.
The apparent color of an object depends not only on its intrinsic spectral reflectivity, but also on the spectrum of the light used to illuminate it. The CIE also has defined a number of standard illuminants which are defined, theoretically, in terms of their spectral content. To completely specify the color of an object, one must measure the XYZ values of the light emanating from the object when it is illuminated by a standard illuminant.
Another CIE color space which is frequently used is the L*a*b* color space. The values of L*, a*, and b* are derived mathematically from the tristimulus values of X, Y, and Z:
                              L          *                =                              116            ⁢                                                  ⁢                                          (                                  Y                                      Y                    n                                                  )                                            1                /                3                                              -          16                                                  a          *                =                  500          ⁡                      [                                                            (                                      X                                          X                      n                                                        )                                                  1                  /                  3                                            -                                                (                                      Y                                          Y                      n                                                        )                                                  1                  /                  3                                                      ]                                                            b          *                =                  200          ⁡                      [                                                            (                                      Y                                          Y                      n                                                        )                                                  1                  /                  3                                            -                                                (                                      Z                                          Z                      n                                                        )                                                  1                  /                  3                                                      ]                              where the values with the subscript “n” are found in published tables and correspond to a chosen standard illuminant. The value of L* is proportional to the brightness (luminosity) of the color. The value of a* describes the red/green composition of the color. The value of b* describes the yellow/blue composition of the color.
The goal of the L*a*b* color space is to provide a color space where the Euclidean distance between color 1 and color 2ΔE=√{square root over ((ΔL*)2+(Δa*)2+(Δb*)2)}{square root over ((ΔL*)2+(Δa*)2+(Δb*)2)}{square root over ((ΔL*)2+(Δa*)2+(Δb*)2)}wherein:    ΔL*=L1*−L2*    Δa*=a1*−a2*    Δb*=b1*−b2*is a “perceptually uniform” measure of the difference between color 1 and color 2. A value of ΔE=1 corresponds to a color difference which is very subtle—so subtle that it would take a trained color observer working under ideal lighting conditions to notice the difference. A value of ΔE=2 corresponds to a difference in color which is twice as noticeable as ΔE=1, and so on. The “perceptual distance” denoted by a given value of ΔE is intended to be independent of the location in color space (that is, independent of hue, saturation, and brightness), but this independence is actually only an approximation. Regardless, ΔE has been accepted in the color industry to quantify color differences.
As stated above, manufacturers typically have used a tristimulus calorimeter, a reflectance spectrophotometer, or a color densitometer to provide more precise color matching by utilizing one or more color measurement systems. These instruments provide quantitative and objective feedback, but are slow and inconvenient, and only measure color at one small spot (typically 5 mm in diameter) at a time, making it inconvenient to impossible to use them to compare all the colors on a complex multi-color pattern. Many colorimeters have to touch the object to get a reading. When trying to do colorimetry on a complex pattern, it is difficult to get the sampling region always in the same spot with respect to the pattern. In addition, these devices tend to be expensive due to the manufacturing care necessary to construct a device capable of providing precise color measurements suitable for laboratory use. These disadvantages make these devices particularly unsuitable for the production floor for use in process control.
Another disadvantage with densitometers is that they do not provide absolute color metrics (such as XYZ tristimulous values). Instead, they report the overall reflectivity of a surface for red, green, and blue light. Color densitometers are only suited for relative (as opposed to absolute) measurements. These relative measurements are often sufficient when the goal is simply to determine if the color on one object “matches” the color on another object.
Therefore there remains a need in the art for a fast and convenient way to efficiently monitor a production process with respect to a standard reference, where the production objects being monitored may have a random spatial orientation, at least around one axis.
Further limitations and disadvantages of conventional, traditional, and proposed approaches will become apparent to one of skill in the art, through comparison of such systems and methods with the present invention as set forth in the remainder of the present application with reference to the drawings.